This invention relates to a novel cGMP-specific phosphodiesterase that provides a method for identifying compounds potentially useful for the treatment and prevention of pre-cancerous and cancerous lesions in mammals.
Cancer and precancer research is replete with publications that describe various biochemical molecules that are over-expressed in neoplastic tissue, leading one research group after another to research whether specfic over-expressed molecules are responsible for the disease, and whether, if the over-expression were inhibited, neoplasia could be alleviated. For example, in familial adenomatous polyposis ("FAP"), Waddell in 1983 (Waddell, W. R. et al., "Sulindac for Polyposis of the Colon," Journal of Surgical Oncology, 24:83-87, 1983) hypothesized that since prostaglandins were over-expressed in such polyps, non-steroidal anti-inflammatory drugs ("NSAIDs") should alleviate the condition because NSAIDs inhibited prostaglandin synthetase (PGE.sub.2) activity. Thus, he administered the nonsteroidal anti-inflammatory drug ("NSAID") sulindac (an inhibitor of PGE.sub.2) to several FAP patients. Waddel discovered that polyps regressed and did not recur upon such therapy. PGE.sub.2 inhibition results from the inhibition of cyclooxygenase (COX) caused by NSAIDs. The success by Waddell and the PGE.sub.2 /COX relationship seemingly confirmed the role of two other biochemical targets--PGE.sub.2 and COX--in carcinogenesis, and the subsequent literature reinforced these views.
Sulindac and other NSAIDs when chronically administered, aggravate the digestive tract where PGE.sub.2 plays a protective role. In addition, they exhibit side effects involving the kidney and interference with normal blood clotting. Thus for neoplasia patients, such drugs are not a practical chronic treatment for FAP. These side effects also prohibit NSAIDs' use for any other neoplasia indication requiring long-term drug administration.
Recent discoveries have lead scientists away from the COX/PEG.sub.2 targets, since those targets do not appear to be the primary (or perhaps even secondary targets) to treat neoplasia patients successfully. Pamukcu et al., in U.S. Pat. No. 5,401,774, disclosed that sulfonyl compounds, that have been reported to be practically devoid of PGE2 and COX inhibition (and therefore not NSAIDs or anti-inflammatory compounds) unexpectedly inhibited the growth of a variety of neoplastic cells, including colon polyp cells. These sulfonyl derivatives have proven effective in rat models of colon carcinogenesis, and one variant (now referred to as exisulind) has proven effective in human clinical trials with FAP patients.
Thus like so many other proteins over-expressed in neoplasias, PGE.sub.2 /COX over-expression is not a cause of neoplasia, rather a consequence. But the discoveries by Pamucku et al., however, have raised the question about how their compounds act --what do such compounds do to neoplastic cells?
Piazza, et al. (in U.S. Pat. No. 5,858,694) discovered that compounds (such as the sulfonyl compounds above) inhibited cyclic GMP, phosphodiesterase, namely, PDE5 and that other such compounds could be screened using that enzyme, which could lead to the discovery of still other pharmaceutical compositions that are anti-neoplastic, and that can be practically devoid of COX or PGE2 inhibition. In addition, anti-neoplastic PDE5-inhibiting compounds can induce apoptosis (a form of programmed cell death or suicide) in neoplastic cells, but not in normal cells. Thus, the conventional wisdom that chemotherapeutics cannot be effective without also killing normal cells is being reversed by such discoveries.
Further, new research presented below has shown that not all compounds exhibiting PDE5 inhibitions induce apoptosis in neoplastic cells. For example, the well-known PDE5 inhibitors, zaprinast and sildenafil, do not induce apoptosis, or even inhibit cell growth in neoplastic cells in our hands, as explained below. However, because pro-apoptotic PDE5 inhibitors induced apoptosis selectively (i.e., in neoplastic but not in normal cells), and many did so without substantial COX inhibition, the usefulness of PDE5 as a screening tool for desirable anti-neoplastic compounds is unquestioned.
However, an even more accurate and selective screening tool than PDE5 to find anti-neoplastic, pro-apoptotic but safe compounds is desirable.